1. Field of the Invention
The invention relates to a cleaning appliance for cleaning washware in a cleaning chamber, with the cleaning appliance having a heat recovery device. The invention also relates to a method for recovering heat in a cleaning appliance. Such cleaning appliances and methods for recovering heat are used, for example, in large-scale catering establishments for cleaning dishes, glasses, cups, cutlery, trays or similar washware. However, other fields of use and types of washware, in particular in the commercial sector, are also feasible.
2. Description of the Background Art
Cleaning appliances by means of which various types of washware can be cleaned with different objectives in view are known in the conventional art. One objective is, for example, that of at least largely freeing the washware from adhering residues of dirt; another objective, which can be realized as an alternative or in addition, is that of sanitizing the washware, which can range as far as disinfecting the washware. Cleaning is generally performed by subjecting the washware to the action of at least one cleaning fluid which can comprise, for example, a liquid cleaning fluid (for example one or more washing liquids, for example water mixed with a detergent and/or a rinse aid) and/or a gaseous cleaning fluid, for example steam.
In many instances, a considerable quantity of thermal energy has to be applied in a cleaning appliance of this type. This thermal energy may be required directly during the cleaning process, for example since the cleaning fluid is applied to washware at an elevated temperature. For example, rinsing liquids having a temperature of approximately 85° C. can be used for a rinsing operation in a dishwasher. A further example is the thermal energy which is required to generate the steam in steam sterilizers and/or steam disinfection appliances. Furthermore, cleaning appliances can also be designed in such a way that one or more drying steps are carried out. In the case of such drying, the washware can be subjected to the action of hot air, for which purpose thermal energy likewise has to be expended.
This outlay on thermal energy can be considerable in the commercial sector in particular, and so, for example, heating capacities can make a considerable contribution to the overall operating costs of the cleaning appliance. In commercial dishwashers, the heating capacities range, for example, from a few 10 kW to a few 100 kW, depending, for example, on the operating state and/or the configuration of the dishwasher.
A further problem in known cleaning appliances, particularly in the field of commercial use, is that they are generally used in a work environment which should not be excessively loaded by waste heat from the cleaning appliance, particularly by damp waste heat. Therefore, considerable outlay is required, for example in large-scale catering establishments, in order to avoid conducting damp waste heat, which is formed in the dishwashers, directly into the work environment, since the working conditions in this work environment would otherwise become unacceptable within a short time. To that extent, for example, complicated on-site exhaust-air devices are required in order to discharge the damp waste heat from the work environment. As an alternative or in addition, the cleaning appliances can have drying apparatuses, in order to draw moisture from the exhaust air and/or to cool the exhaust air.
Numerous drying apparatuses which assist drying of the washware and fundamentally also dehumidify the exhaust air emitted into the surrounding area are known from the prior art. One drying concept which is known from the prior art involves equipping cleaning appliances with what are known as latent heat accumulators. Latent heat accumulators are devices which can store thermal energy by utilizing the enthalpy of reversible thermodynamic state changes in an accumulator medium (called heat accumulator materials in the text which follows). A typical example of such reversible thermodynamic state changes are, for example, phase transitions (for example melting/solidification) or reversible chemical reactions, for example to absorption and desorption processes which are based on chemical sorption, or processes in which a crystallization phase transition takes place. One example of a domestic dishwasher in which a thermally insulated heat exchanger with a latent heat accumulator is used is described in DE 37 41 652 A1. Other examples of such domestic appliances comprising similar apparatuses with latent heat accumulators can be found in DE 37 00 567 A1 (a baking oven in that document), in DE 10 2005 012 114 A1 (a dishwasher) or in DE 196 22 882 C2 (again a dishwasher). DE 29 16 839 describes, in general, a method for extracting thermal energy from hot liquids, in which method the liquids flow in succession through a plurality of latent heat accumulators which are operated at different temperature levels.
A further example of a drying principle is drying apparatuses which operate with the aid of Peltier elements. An example of a drying apparatus of this type is known from DE 198 13 924 A1. This document describes a condensation device for a domestic appliance, comprising a module element with a Peltier element. The Peltier element has a heat-absorbing surface and a heat-emitting surface. The heat-absorbing surface draws heat from a work space atmosphere of a work space of the domestic appliance, as a result of which moisture from the work space atmosphere condenses at the cooled location and therefore a drying operation of the domestic appliance is more effective and quicker. The heat-emitting surface of the Peltier element can also be coupled to a heat-absorbing volume, for example a water container.
However, from the point of view of commercial applicability, the apparatus described in DE 198 13 924 A1 has the disadvantage that, if the Peltier element heats up to too great an extent, the water container for cooling said Peltier element has to be emptied and filled with fresh water. To that extent, on the one hand, the functionality of the condensation device is unstable and can fluctuate over a relatively long operating time. This can be a considerable disadvantage, particularly in commercial cleaning appliances which, for example, have to operate continuously for several hours. Moreover, a safe and reliable drying operation is not ensured in all instances because of the described temperature drift in the water container. Furthermore, the energy contained in the waste heat is lost, and even additional energy has to be expended in order to operate the Peltier element.
Cooling appliances are also known from the field of air-conditioning technology, in which cooling appliances Peltier elements for conditioning room air and other media are used. Therefore, for example, EP 0 842 382 B1 describes a compact H-thermal appliance which comprises thermocouple blocks having a plurality of Peltier elements. In this case, thermal energy is transferred from a medium on a cold side to a medium on a hot side. In this case, it is proposed, inter alia, to collect the hot water which is produced as service water and make it available for further use. Overall, however, the design described in EP 0 842 382 B1 is comparatively complex.
Cleaning appliances are known from the field of commercial dishwashers, which cleaning appliances not only attempt to mitigate the described problem of loading the surrounding area with exhaust air, but are also designed to allow at least partial heat recovery of the thermal energy contained in the waste heat. One example of systems of this type is illustrated in U.S. Pat. No. 3,598,131. In this case, steam is removed from a dishwasher by suction by means of a suction-removal apparatus and passed into a shaft and is conducted via a heat exchanger. The heat exchanger is in this case configured as porous material which is sprayed with fresh water. The condensed moisture is collected and is fed back to the dishwasher. A similar dishwasher with heat recovery is also illustrated in DE 10 2004 003 797 A1, which corresponds to U.S. Publication No. 20070131260.
However, the disadvantage of the cleaning appliance illustrated in U.S. Pat. No. 3,598,131 is that the functionality of the heat recovery device depends greatly on the temperature of the cold water sprayed on. If the dishwasher is operated, for example, in regions with a hot climate, then the “cold water” is usually at a temperature different to that in regions with a milder or even cool climate. To that extent, the functionality of the heat recovery device can fluctuate sharply, and selective dehumidification or cooling cannot be ensured in all cases.
A further disadvantage of the heat recovery device described in U.S. Pat. No. 3,598,131 is that cooling liquid mixes with the condensed water, so that, overall, the recirculated water is at a comparatively low temperature and generally has to be reheated before it can be fed to the cleaning operation again. Moreover, the described heat recovery device has disadvantages in hygiene terms, since there is a risk of bacteria growing in the condensed water and therefore in the washware or the porous heat exchanger.
A further disadvantage of known heat recovery devices, for example the devices described in U.S. Pat. No. 3,598,131 or in DE 10 2004 003 797 A1, is that apparatuses of this type cannot operate satisfactorily in all operating states. Therefore, what is known as a standby mode, that is to say a mode in which the cleaning device is made and kept ready for operation without the cleaning process actually being carried out, is provided, particularly in commercial dishwashers, in many cases. During this time, at least one fluid tank for a cleaning fluid is usually heated in order to establish or maintain readiness for operation. This results in the production of steam vapors which generally have to be dispelled from the cleaning device in order to prevent an overpressure. Therefore, vapors and waste heat which could load the ambient air would be produced as early as in the standby mode. However, the known heat recovery devices are suitable to a limited extent for eliminating these vapors since no fresh water which could absorb the heat and feed it back to the cleaning device flows in the standby mode. However, additional consumption of fresh water would increase the operating costs. Heat recovery in the standby made would in any case be ended at the latest when one or more fluid tanks of the cleaning appliance is/are filled with heated water, since in this case the heated water would have to be disposed of in an outlet, and this would again waste the recovered energy.
Furthermore, it may be possible to use what are known as heat pumps to recover heat. Heat pumps are machines which, with provision of mechanical work, pump heat from a low temperature level to a higher temperature level. This can, in particular, counteract the problem of cooling water, after flowing through the heat recovery device, being at a comparatively low temperature and having to be heated up further after being recirculated to the cleaning appliance. In heat pumps, heat of evaporation is generally utilized in order, for example, to draw a quantity of heat from the waste heat of a dishwasher. However, heat pumps generally cannot be regulated as required, and, in practice, are restricted in terms of their control behavior, since only two-step control is possible. Moreover, the heat pumps have a defined operating point with a fixed tolerance which is not scalable. This presents problems in many instances, particularly for commercial use. Moreover, the use of heat pumps is in most cases associated with considerable additional costs and allowing for considerable installation spaces. Further disadvantages of the use of heat pumps include the noise occurring during operation, the high mechanical wear and vibrations.